In situ study of oxidation states and structure of 4 nm CoPt bimetallic nanoparticles during CO oxidation using X-ray spectroscopies in comparison with reaction turnover frequency

doi:10.1016/j.cattod.2011.10.009

Catalysis Today

Volume 182, Issue 1, 17 March 2012, Pages 54-59

https://doi.org/10.1016/j.cattod.2011.10.009 Get rights and content

Abstract

In situ near edge X-ray absorption fine structure (NEXAFS) spectroscopy and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) were performed to monitor the oxidation state and structure of 4 nm CoPt nanoparticles during the reaction of CO with O₂ – a model oxidation reaction. In addition, reversible changes in the oxidation state of cobalt as a function of cycling CO and O₂ pressure (in the range of millitorr to 60 Torr) were quantified and compared. Turnover frequency reaction data was also obtained for the CoPt nanoparticles and correlated with the oxidation states and structures observed spectroscopically. These findings indicate that separated from the effect of partial pressure of the reactant gases, chemical state and structure changes of the CoPt nanoparticles during CO oxidation are important factors in determining the rate of the reaction.

Graphical abstract

Highlight

► In situ NEXAFS characterization of 4 nm CoPt nanoparticles under the cycling total pressure (up to 60 Torr) and reaction temperature of the reactant gases. ► Ambient pressure XPS shows evidence of surface segregation of Co in the CO oxidation condition. ► Turnover frequency is correlated to the AP-XPS and in situ NEXAFS results.

Introduction

Cobalt is well known for its use in the catalytic hydrogenation reactions of CO and CO₂ to produce gaseous or liquid hydrocarbons, with a long history of producing synthetic fuels. Here we consider its role in the model oxidation reaction of CO to CO₂. By choosing to explore model reactions and carefully characterizing the catalyst under reaction conditions it is hoped that we can gain a greater understanding of the fundamental factors that determine catalyst behavior. A significant number of previous studies have been conducted on CO oxidation of Co oxide or Co–Pt bimetallics [1], [2], [3], [4], [5], [6], [7], [8], [9]. In these studies it has been suggested that the oxidation state of Co plays a critical role in the catalytic reaction mechanism: specifically, cobalt oxide provides oxygen to combine with the adsorbed CO molecules and Co is itself then re-oxidized to provide a new active site.

Bimetallic CoPt nanoparticles have recently drawn attention in many areas of catalysis in a quest to reduce precious metal content while maintaining optimum catalytic reactivity. Stamenkovic et al. [10] have documented a family of Pt and 3d transition metal alloy catalysts that are more effective in oxidizing CO than monometallic Pt. Likewise, bimetallic CoPt nanoparticle catalysts have recently found application in reforming small sugar molecules in water [11]. CO oxidation is a model reaction that has been studied extensively by us and others. The goal of our present work is therefore to understand the role of oxidation state and elemental composition changes of one such typical bimetallic catalyst's surface reactivity. To achieve this, we embarked on a combined catalytic and in situ spectroscopic study of bimetallic 4 nm CoPt nanoparticles used as catalysts for CO oxidation in the 10 mTorr–100 Torr pressure range and at 125 °C.

We previously [12] studied the evolution of the cobalt oxidation state under reducing and oxidizing conditions for 4 nm Co and CoPt nanoparticles. We found that the presence of platinum facilitates the rapid reduction of cobalt oxides in H₂ at low temperature (38 °C) – the reduction happens much more easily than occurs for pure Co. In addition, reversible changes of the oxidation state of cobalt in the nanoparticles as a function of reactant pressure (in the range of millitorr to 36 Torr) were quantified and compared. Using in situ X-ray spectroscopies we report on exploring a real catalytic reaction – CO oxidation with 4 nm CoPt nanoparticles, as a function of reaction pressure. These spectroscopic results are then correlated to turnover frequency measurements under an identical range of conditions.

Section snippets

Materials

Pt(acac)₂, Co(acac)₂, polyvinylpyrollidone (PVP, M_w ∼ 55,000), benzyl alcohol and 1,6 hexanediol were purchased from Aldrich. Ethylene glycol, acetone, ethanol (absolute) were purchased from VWR. All chemicals were used as received.

4 nm CoPt alloy catalyst was synthesized according to a methodology which was recently reported [12]. Briefly, Pt(acac)₂ (acac = acetylacetonate) and Co(acac)₂ precursors were co-nucleated in ethylene glycol in the presence of PVP as a capping agent in an oil bath at 240

Results and discussion

Employing X-ray spectroscopies allows exploration of the catalysts’ real structures under reaction conditions – in particular we have adopted two synchrotron based methodologies to achieve this goal: firstly Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy and secondly Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS). Fig. 2 shows a sequence of NEXAFS spectra acquired for the 4 nm CoPt alloy catalyst under the cycling pressures of reactant gases. As described previously

Conclusions

To summarize in situ X-ray spectroscopy measurements and catalytic turnover frequency measurements were performed on 4 nm CoPt alloy nanoparticles and the fractions of Co oxidation states mapped while cycling the total pressure of O₂, CO and He between a few millitorr and one bar total pressure. Ambient pressure XPS indicates a significant change in the elemental distribution within each bimetallic particle, changing from Pt covered surfaces under reducing conditions to a significant surface

Acknowledgements

This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors also acknowledge support of the National Center for Electron Microscopy, Lawrence Berkeley Lab, which is supported by the U.S. Department of

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¹: These authors contributed equally to this work.

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In situ study of oxidation states and structure of 4 nm CoPt bimetallic nanoparticles during CO oxidation using X-ray spectroscopies in comparison with reaction turnover frequency

Abstract

Graphical abstract

Highlight

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Applied Catalysis B: Environmental

Applied Catalysis B: Environmental

Journal of Catalysis

Journal of Catalysis

Journal of Catalysis

Journal of Catalysis

Journal of Catalysis

Applied Catalysis B: Environmental

Journal of Catalysis